Nine-Residue Peptide Self-Assembles in the Presence of Silver to Produce a Self-Healing, Cytocompatible, Antimicrobial Hydrogel

Silver compounds have been used extensively for wound healing because of their antimicrobial properties, but high concentrations of silver are toxic to mammalian cells. We designed a peptide that binds silver and releases only small amounts of this ion over time, therefore overcoming the problem of silver toxicity. Silver binding was achieved through incorporation of an unnatural amino acid, 3'-pyridyl alanine (3'-PyA), into the peptide sequence. Upon the addition of silver ions, the peptide adopts a beta-sheet secondary structure and self-assembles into a strong hydrogel as characterized by rheology, circular dichroism, and transmission electron microscopy. We show that the resulting hydrogel kills Escherichia coli and Staphylococcus aureus but is not toxic to fibroblasts and could be used for wound healing. The amount of Ag(I) released by hydrogels into the solution is less than 4% and this low amount of Ag(I) does not change in the pH range 6-8. These studies provide an initial indication for use of the designed hydrogel as injectable, antimicrobial wound dressing.

Ratiometric Phosphorescent Silver Sensor: Detection and Quantification of Free Silver Ions within Silver Nanoparticles

Silver nanoparticles (AgNPs) have well-known antibacterial properties that have stimulated their widespread production and usage, which nonetheless concomitantly raises concerns regarding their release into the environment. Understanding the toxicity of AgNPs to biological systems, the environment, and the role that each silver species (Ag⁺ ions vs AgNPs) plays in that toxicity has received significant attention. One of the critical objectives of this research is the development of a reliable method that can sense and differentiate free silver ions from AgNPs and is able to characterize silver ions leaching from nanosilver. A number of analytical methods described in the literature that are available for sensing silver ions are costly, time consuming, tedious, and, more importantly, destroy the AgNP sample. To address these issues, a phosphorescent gold(I)-pyrazolate cyclic trinuclear complex (AuT) known to detect free silver ions was employed to detect and differentiate silver ions from AgNPs within an AgNP sample. The advantage of the proposed silver sensor is its ratiometric emission capability that undermines any background interference. The sensor exhibits a strong red emission (λ_max = ∼690 nm) that, in the presence of Ag⁺ ions, will form a bright-green emissive adduct with a blue-shifted peak maximum near 475 nm yet red-shifted excitation peak. The presence of AgNPs did not inhibit the silver detection and quantification ability of the phosphorescent silver sensor. To understand the chemical transformation of nanosilver, the leaching of silver ions from AgNPs over a period of 35 days was monitored and quantified by measuring the I/ I_o changes of the sensor. Furthermore, through adduct formation, the AuT molecular system was able to remediate free silver ions from the solution. The stronger affinity of the AuT complex to "sandwich" free silver ions than AgNPs was demonstrated in the presence of KCl salt that is well documented to form AgCl in the presence of silver ions. To our knowledge, this is the only ratiometric luminescence-based silver sensor able to successfully differentiate between Ag⁺ ions and AgNPs, sense the silver leakage from AgNPs, and remediate toxic silver ions from an aqueous solution. The synthesis and characterization of this sensor is a simple, single-step process-anticipating its viability for various applications.

AgNPs Change Microbial Community Structures of Wastewater

Due to their strong antimicrobial activity, silver nanoparticles (AgNPs) are massively produced, applied, consumed and, as a negative consequence, released into wastewater treatment plants. Most AgNPs are assumed to be bound by sludge, and thus bear potential risk for microbial performance and stability. In this lab-scale study, flow cytometry as a high-throughput method and 16S rRNA gene amplicon Illumina MiSeq sequencing were used to track microbial community structure changes when being exposed to AgNPs. Both methods allowed deeper investigation of the toxic impact of chemicals on microbial communities than classical EC₅₀ determination. In addition, ecological metrics were used to quantify microbial community variations depending on AgNP types (10 and 30 nm) and concentrations. Only low changes in α- and intra-community β-diversity values were found both in successive negative and positive control batches and batches that were run with AgNPs below the EC₅₀ value. Instead, AgNPs at EC₅₀ concentrations caused upcoming of certain and disappearance of formerly dominant subcommunities. Flavobacteriia were among those that almost disappeared, while phylotypes affiliated with Gammaproteobacteria (3.6-fold) and Bacilli (8.4-fold) increased in cell abundance in comparison to the negative control. Thus, silver amounts at the EC₅₀ value affected community structure suggesting a potential negative impact on functions in wastewater treatment systems.

Using a Chemical Genetic Screen to Enhance Our Understanding of the Antibacterial Properties of Silver

It is essential to understand the mechanisms by which a toxicant is capable of poisoning the bacterial cell. The mechanism of action of many biocides and toxins, including numerous ubiquitous compounds, is not fully understood. For example, despite the widespread clinical and commercial use of silver (Ag), the mechanisms describing how this metal poisons bacterial cells remains incomplete. To advance our understanding surrounding the antimicrobial action of Ag, we performed a chemical genetic screen of a mutant library of Escherichia coli—the Keio collection, in order to identify Ag sensitive or resistant deletion strains. Indeed, our findings corroborate many previously established mechanisms that describe the antibacterial effects of Ag, such as the disruption of iron-sulfur clusters containing proteins and certain cellular redox enzymes. However, the data presented here demonstrates that the activity of Ag within the bacterial cell is more extensive, encompassing genes involved in cell wall maintenance, quinone metabolism and sulfur assimilation. Altogether, this study provides further insight into the antimicrobial mechanism of Ag and the physiological adaption of E. coli to this metal.